Toroidal phase shifters have been deployed for many decades. Toroidal phase shifters are formed by one or more portions of ferrite in a waveguide. Toroidal phase shifters may also be referred to as ferrite phase shifters. Phase shift is affected by the magnetized moments in the magnetized ferrite interacting with electromagnetic fields propagating through the waveguide. For latching ferrite phase shifters, latch wire(s) are placed through the center of the ferrite portion(s). Current flowing through the latch wire(s) is used to adjust the magnetized moments in the ferrite, and thus the amount of phase shift induced in electromagnetic fields propagating through the toroidal phase shifter.
Toroidal phase shifters formed in rectangular waveguide typically operate using an LSE10 electromagnetic mode. LSE is an acronym for longitudinally section electric. LSE electromagnetic modes occur in dielectrically loaded rectangular waveguides. Else, the modes are transverse electric (TE) modes. Electromagnetic mode may hereinafter be referred to a “mode”.
The transition between the rectangular waveguide and the ferrite portion(s) (and possibly other materials such as dielectric(s) in the core of the ferrite portion(s)) is designed to reduce reflections, and thus return loss, for the LSE10 mode. Depending upon design of the toroidal phase shifter, e.g. waveguide and ferrite portion(s) design, the number of undesirable modes above waveguide cutoff frequency may vary.
A narrow band toroidal phase shifter can be designed so that the cutoff frequencies of some or all of the undesirable higher order modes are higher than the frequency band of interest, thus eliminating the undesirable higher order modes below the cut off frequencies. This may not possible or practical when designing a toroidal phase shifter to operate over a very broad bandwidth, e.g. at least one half of an octave bandwidth. The rectangular waveguide of a broadband toroidal phase shifter supports the propagation of two or more higher order modes, e.g. at least the LSE11 and LSE01 modes, and possibly the LSE20 mode—where each of these modes typically has a successively higher cut off frequency.
Because the transition between the rectangular waveguide and the ferrite portion(s) (and possibly other materials such as dielectric(s) in the core of the ferrite portion(s)) can not readily be designed to reduce reflections for undesirable higher order modes, the electromagnetic waves of the undesirable higher order modes are reflected. The reflections of the undesirable higher order modes cause resonances in the operating bandwidth of the toroidal phase shifter. The insertion loss and phase shift of the toroidal phase shifter at the resonant frequencies are dramatically changed, e.g. increased. As a result, the insertion losses and phase shifts at the resonances no longer are within desired parameter ranges. Furthermore the phase characteristics at the resonant frequencies are significantly impacted in such a way that may be detrimental to the system performance.
Resistive film imbedded in a center dielectric inside or between the ferrite toroid(s), and/or a dielectric transformer may be used to suppress the first undesirable higher order mode, the LSE11 mode, in a toroidal phase shifter. However, the next undesirable higher order modes, such as the LSE01 and LSE20 modes, typically need to be suppressed in the very broad band toroidal phase shifter.
Therefore, there has been a need for many decades for an approach to suppress the next undesirable higher order modes, such as the LSE01 and LSE20 modes, in broad band toroidal phase shifters.